Legal requirements which are becoming ever stricter with respect to permissible emissions of pollutants by internal combustion engines for motor vehicles make it necessary to carry out improved mixture preparation in the cylinders of the internal combustion engines by means of fuel injectors. In contemporary fuel injectors, control of the injection of fuel is carried out by means of a nozzle needle which is displaceably mounted in the fuel injector and opens or closes an opening cross section or an injection hole or a multiplicity thereof of a nozzle assembly of the fuel injector as a function of its stroke. The nozzle needle is actuated, for example, by means of a piezo-electric actuator which activates the nozzle needle hydraulically or mechanically.
In order to lower the emissions of pollutants by the internal combustion engine and to keep the consumption of said internal combustion engine as low as possible, it is desirable to achieve the best possible combustion within the cylinders of the internal combustion engine. For good process control or open-loop/closed-loop control of the combustion process in the cylinders it is necessary to be able to carry out the most precise metering possible, in terms of volume and timing, of the fuel which is to be injected, in order to achieve the best possible combustion and/or most complete possible regeneration of a particle filter of the motor vehicle at any time, since torque requirements of the internal combustion engine are converted into injection quantities which in turn correlate with an injection duration as a function of an injection pressure, a stroke of the nozzle needle and/or geometry of the fuel injector.
Deviation of an actual injection quantity from a setpoint injection quantity of a fuel injector always has adverse effects on a combustion process, that is to say on the emissions of pollutants which arise as a result, and usually also has adverse effects on the consumption of the internal combustion engine. In particular, for fuel injectors which inject directly, stringent requirements apply with respect to the accuracy of the injection quantities and the stability of a jet pattern under all operating conditions and over the entire service life of the fuel injector. This applies even more with respect to small injection quantities, in a multiple-injection mode with the associated short injection intervals and/or in a partial stroke mode of a nozzle needle.
An injection nozzle of the fuel injector is actuated by the nozzle needle which can be driven, for example, by means of a servo valve which can be actuated by means of a piezo actuator. A nozzle needle which is driven hydraulically indirectly in such a way is the state of the art. However, the nozzle needle can also be actuated directly without a detour via a servo valve. In such a fuel injector, coupling of the movement of the piezo actuator, and subsequent thereto, movement of the nozzle needle can take place hydraulically directly, which provides significant advantages. The same requirements apply to such hydraulically directly driven fuel injectors as to injection nozzles which can be actuated by means of a servo valve. However, further advantageous properties of the fuel injectors arise as a result of a hydraulic direct drive.